In recent years, there has been an increasing demand for global shutters using CMOS image sensors for imaging for industrial production lines and for imaging subjects moving at high speed.
In a CMOS image sensor, a plurality of pixels share one reading circuit, and sequential reading is performed for each pixel. At that time, a so-called rolling shutter is adopted. The rolling shutter is a system in which exposure accumulation synchronized with reading is performed for each pixel. Therefore, different exposure times are obtained for each pixel included in one frame.
In such a rolling shutter, it is known that so-called focal plane distortion occurs in a subject moving at high speed. Moreover, in a case where pulsed light is detected by the CMOS image sensor, one pulse emission is divided into two frames and output with the pixel being read at that time as the boundary, and detection is difficult.
Thus, the adoption of global shutters which align exposure times with all effective pixels is increasing. Mainly, in such global shutters, exposing is started simultaneously with electronic shutters, and after the exposure is completed simultaneously, accumulated charges are temporarily stored in floating diffusion, which is an input of a pixel amplifier, until reading for each pixel is performed.
As compared with a rolling shutter that can continuously read a reset signal and an accumulated signal and perform correlated double sampling (CDS) between the signals, the global shutter is hard to control an output offset for each pixel after the reset.
There is a fixed offset component due to the characteristic variation of elements in the output of each pixel, and furthermore, if the floating diffusion is reset, random kTC noise is generated for each pixel.
In the rolling shutter, at the time of reading of the accumulated signal, after the reset of the floating diffusion of the pixel is performed, auto zero is applied to a comparator and an amplifier of the subsequent stage, so that the offset can be easily canceled for each pixel. Therefore, if the accumulated signal is continuously read and subjected to AD conversion thereafter, the output offset due to the kTC noise of the pixel or the characteristic variation has already been removed.
On the other hand, in the global shutter, it is impossible to continuously read the reset level and the accumulation level in the same pixel, and therefore, it is impossible to perform such offset cancellation for each pixel.
As one of countermeasures, for example, Patent Document 1 proposes a method of resetting a pixel again after reading an accumulated signal and canceling a fixed offset by taking a difference from a reset signal.
Furthermore, as a method for performing cancellation including the kTC noise for each pixel, Patent Document 2 proposes reading twice before and after the start of exposure. In this proposal, reset is first performed before the start of exposure, and a reset signal for each pixel is acquired for all effective pixels and is stored in a memory or the like as digital data. Those are subtracted from the accumulated signals acquired after completion of exposure, and the CDS is performed.